An amplifier, electronic amplifier or (informally) amp, is an electronic device that can increase the power of a signal (a time-varying voltage or current). An amplifier uses electric power from a power supply to increase the amplitude of a signal. The amount of amplification provided by an amplifier is measured by the amplifier's gain: the ratio of output signals to input signals. An amplifier can provide a power gain greater than one.
Amplifiers have a number of performance metrics related to gain. In particular, power-added efficiency (PAE) is a metric for rating the efficiency of a power amplifier that takes into account the effect of the gain of the amplifier. PAE is calculated as a percentage. Additionally, most amplifiers maintain a relatively constant gain for low-level input signals. However, at higher input signal levels, amplifiers go into saturation and that decreases the amplifier's gain, thereby causing non-linearity in the amplifier, which can be referred to as compression. The one decibel (1 dB) compression point (P1 dB) indicates the power level that causes the gain to drop by 1 dB from the amplifier's small signal value.